common/lcd.c: cleanup use of global variables

lcd_color_fg and lcd_color_bg had to be declared in board specific
code, but were not actually used there; in addition, we have getter /
setter functions for these, which were not used either.

Get rid of the global variables, and use the getter function where
needed (so far no setter calls are needed).

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Alessandro Rubini <rubini@unipv.it>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Bo Shen <voice.shen@atmel.com>
Cc: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stelian Pop <stelian@popies.net>
Cc: Tom Warren <twarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Jeroen Hofstee <jeroen@myspectrum.nl>
[agust: also fixed cm_t35 board while rebasing]
Signed-off-by: Anatolij Gustschin <agust@denx.de>